Windows Vista has a 30-day grace period before you’re required to activate it. This can be inconvenient for people who do a lot of testing and frequent clean installs; even though the MSDN subscription includes several activations, what’s the point of activating it on a real or virtual machine that may be running some other operating system in 31 days? Neowin.net explains the official way to get a little extra time in your grace period. Link.